Mechanistic aspects of facet-dependent CH4/C2+ selectivity over a χ-Fe5C2 Fischer–Tropsch catalyst

Structure–performance relationship is a complex issue in iron-catalyzed Fischer–Tropsch synthesis, and it is not easy to elucidate it by experimental investigations. First-principle calculation is a powerful method for explaining experimental results and guiding catalyst design. In this study, we investigated the reaction mechanisms of CH4 formation and C–C coupling on four χ-Fe5C2 surfaces and established the kinetic equations to compare the rates of CH4 formation and C1+C1 coupling reactions and determine the CH4/C2+ selectivity. The results show that the geometry of the χ-Fe5C2 surfaces has little effect on the formation rate of CH4; however, the C1+C1 coupling reactions are significantly affected by the surface geometry. The C1+C1 coupling reaction rates on the terraced-like (510) and (021) surfaces are much higher than those on the stepped-like (001) and (100) surfaces. Based on these results, we established a Brønsted–Evans–Polanyi (BEP) relationship between the effective barrier difference for CH4 formation and C1+C1 coupling (ΔEeff) and the adsorption energy of C+4H (ΔEC+4H) on χ-Fe5C2 surfaces. ΔEC+4H can be used as a descriptor for CH4/C2+ selectivity on different surfaces of χ-Fe5C2.